Techniques for conventional phase-change type rewritable optical recording media are as follows.
Those optical recording media have a recording layer consisting mainly of telluride, which, for recording, is partially melted by applying a converged pulsed laser beam to the crystalline-state recording layer for a short period of time. The melted portion is rapidly cooled and solidified due to thermal diffusion, resulting in the formation of an amorphous record mark. With an optical reflectivity lower than that for the crystalline state, the record mark can read a signal.
To erase the data, a laser beam is applied to the record mark to heat the recording layer up to a temperature that is lower than the melting point but higher than the crystallization temperature so that the amorphous record mark portion is crystallized to allow that portion of the layer to recover the unrecorded state.
The known materials for the recording layer of such phase-change type rewritable optical recording media include some alloys such as Ge.sub.2 Sb.sub.2 Te.sub.5 (N. Yamada et al., Proc. Int. Symp. on Optical Memory 1987 pp.61-66).
These optical recording media with its recording layer consisting of a Te alloy has a high crystallization rate to allow high-speed overwriting to be performed by varying the power of the beam which has a circular cross-section. In the optical recording media with such a recording layer, a heat-resistant transparent dielectric layer is provided on both sides of the recording layer to prevent deformation and opening formation from occurring on the recording layer during the recording process. In addition, there are other known techniques which use a reflecting metal layer, of Al etc., provided over the dielectric layer so that some optical interference is caused to improve the signal contrast during reading and so that the recording layer is cooled efficiently to facilitate the formation of amorphous record marks and to improve the erasing characteristics and repeated use characteristics.
In particular, it has been known that a structure where the dielectric layer between the recording layer and the reflection layer is about 50 nm or less in thickness (rapid cooling disk structure) is small in the variation of recording characteristics due to repeated erasing and writing and wide in erasing power margin as compared to ones with a dielectric layer with about 200 nm or more thickness (thick second dielectric layer structure)(T. Ohta et al. Japanese Journal of Applied Physics, Vol.28 (1989) Suppl.28-3, pp123-128).
These conventional phase-change type rewritable optical recording media have such problems as follows:
In the case of the conventional structure, the shape and position of an overwritten record mark is affected by the mark recorded before the overwriting, resulting in limits to the erasing rate and jitter characteristics. In particular, said problems may worsen when the data density for pit position recording is increased by, for example, using short-wave laser to reduce the size of optical spots, or when the data density for mark-edge recording, instead of conventional pit position recording, is increased by, similarly to the case of pit position recording, using short-wave laser to reduce the size of optical spots, or when data are recorded at a high linear-velocity.
This may be attributed to the fact that the difference of the reflectance between the crystalline portions and the amorphous record mark portions is so large that the light absorption by the amorphous portions of the recording layer becomes larger than that by the crystalline portions, causing the record mark portions that carry data to be heated more rapidly during the recording process. Thus, the heating of a portion during the recording process becomes dependent on whether the portion is crystalline or amorphous before the overwriting. This may results in the fact that the shape and position of the record marks in an overwritten portion are affected by the mark existing before overwriting to impose limits to the erase ratio and jitter characteristics. With the conventional rapid cooling disk structure, in particular, a layer of aluminum, gold, etc., with a high thermal conductivity and high reflectance is provided on the dielectric layer on that side which is not exposed to the incident light in order to obtain high durability and good recording characteristics. When such a high-reflectance layer is provided, however, it is difficult to solve the problem that the light absorption by the amorphous portions of the recording layer becomes larger than that by the crystalline portions.
If the distances between record marks are decreased to less than about the size of the incident light beam (.lambda./NA) in an attempt to increase the recording density, limitations associated with the optical resolution will reduce the amplitude of read signals. If, in particular, data for which the distances between record marks are decreased to less than about the size of the incident light beam (.lambda./NA) are overwritten, the shape and position of the record marks are affected by the marks existing before the overwriting to impose limits to the erase ratio and jitter characteristics, which may also be a factor in this problem.
To solve these problems, the technique described below has been known as a means to prevent the light absorption by the amorphous portions from becoming larger than that by the crystalline portions. That is, as proposed in the Patent Laid-Open (Kokai) HEI 5-159360, a second dielectric layer with a 220 nm thickness is formed first, and then a Ti layer for light absorption with a 50 nm thickness is formed, followed by the formation of a relatively thin Al layer for heat radiation with an about 50 nm thickness to reduce the thermal load imposed on the light absorbing layer due to the absorption.